1. Field of the Invention
This invention relates to a new process for the production of isocyanurate polyisocyanates containing urethane and ester groups (isocyanurate polyisocyanates) to the compounds obtained by this process and to their use as the polyisocyanate component in the production of polyurethane plastics and more particularly in two-component polyurethane coating compositions.
2. Description of the Prior Art
Aliphatic isocyanurate polyisocyanates, particularly those based on hexane-1,6-diisocyanate (hereinafter referred to as "HDI") have acquired industrial significance. They are used primarily as the polyisocyanate component in two-component polyurethane coating compositions and also for the production of moisture-curing one-component polyurethane binders or, after blocking with blocking agents for isocyanate groups, in heat-crosslinkable polyurethane coating compositions. These coating compositions are mainly used for painting inflexible substrates, such as metals and wood, and are distinguished by their excellent light stability, weathering resistance, hardness and adhesion. HDI-based isocyanurate polyisocyanates may be distinguished from corresponding biuret types, which are also industrially used, by greatly improved resistance to yellowing and chemicals, for example, resistance to tar stains.
The chemical bases of the various polyurethane coating compositions are described inter alia in "Lackkunstharze" by Hans Wagner and Hans Friedrich Sarx, Carl Hanser Verlag, Munchen 1971, pages 153 to 173 and in "Lehrbuch der Lacke und Beschichtungen", Vol. I, Part 2, by Hans Knittel, Verlag W. A. Colomb, Berling-Oberschwandorf 1973, pages 512 to 612.
The production of isocyanurate polyisocyanates is described, for example, in GB-PS 920,080, DE-AS 1,667,309, DE-OS 3,100,262, DE-OS 3,219,608, DE-OS 3,240,613, DE-OS 3,811,350, EP-A 10,589, EP-A 57,653, EP-A 89,297, EP-A 187,105 and Japanese patent application Sho 59-271970 of Dec. 25, 1984 published as Sho 61-151179 on Jul. 9, 1986. The use of substoichiometric quantities of hydroxyl compounds is also mentioned in some of these prior publications. For example, DE-AS 1,667,309 describes the production of isocyanurate polyisocyanates using hydroxyl compounds as co-catalysts. DE-OS 3,219,608 describes the use of polyhydric alcohols having a molecular weight below 3,000 in a quantity of up to 15 mole-%, based on HDI, in the production of isocyanurate polyisocyanates based on HDI. The polyhydric alcohols which are said to be suitable in this prior publication also include polyester polyols which are not specified in detail. Low molecular weight diols bearing lateral alkyl groups are used as modifiers in the process according to EP-A-155,559. The above Japanese publication recommends the use of alkanols having from 6 to 9 carbon atoms as cocatalyst for the trimerization reaction.
The modification of the starting diisocyanate with certain diols containing ester groups in accordance with DE-OS 3,811,350 is intended in particular to increase the elasticity of coatings produced from the modified polyisocyanates, a certain improvement in the compatibility of the modified polyisocyanates also being obtained.
However, one feature common to all known processes for the production of isocyanurate polyisocyanates based on aliphatic diisocyanates, particularly HDI, is that the end products do not entirely satisfy practical requirements in regard to low viscosity and compatibility with hydroxyl-containing paint resins and physiologically substantially safe solvents. In addition, the production of particularly low viscosity isocyanurate polyisocyanates based on HDI has previously only been possible at particularly low conversions of about 10 to 20% of the HDI used. Processes in which the trimerization reaction has to be terminated at a very low degree of trimerization and in which the excess HDI has to be removed by distillation are naturally complicated and expensive.
Accordingly, an object of the present invention is to provide new isocyanurate polyisocyanates based on aliphatic diisocyanates which have both a low viscosity and improved compatibility with known hydroxyl-containing paint resins and known physiologically safe solvents. It is an additionally of the present invention to provide new isocyanurate polyisocyanates which may be produced in an economically advantageous manner at a high conversion of the diisocyanate used.
These objects may be achieved by the process according to the present invention which is described in detail hereinafter.